Powder coating systems are currently widely used to apply a protective coating to a variety of articles. These systems generally comprise a powder spray booth which provides a controlled area for the application of powder material onto articles which are transmitted through the booth on a conveyor past one or more automatically manipulated or manually operated powder spray guns. In most systems of this type, an electrostatic charge is applied to the powder coating by the spray guns, with the articles to be coated being held at opposite or ground potential, to enhance the quantity of powder which attaches to the articles and to assist in retaining the powder on the articles. After passing through the powder spray booth, the coated articles are then conveyed to an oven wherein the powder is heated so that it melts, and when subsequently cooled, forms a solid continuous coating on the articles.
Although the electrostatic charge applied to the powder coating material enhances its adherence to the articles to be coated, it is not uncommon for up to half of the total powder sprayed to fail to attach to the articles. This unattached or nonadhered powder material is referred to as "overspray" which must be collected and recycled in order to make the powder coating operation economically feasible. Collection of the oversprayed powder material is conventionally achieved by entraining the oversprayed powder in an airstream withdrawn from the powder booth, conveying that powder entrained airstream to a powder recovery system and there separating the powder from the air so that it either can be stored or recirculated back to the spray guns for reuse. One type of powder recovery system employs cartridge filters which communicate with the interior of the booth, and a fan or blower unit which creates a negative pressure within the booth interior so that the air-entrained powder material is drawn to the cartridge filters within a powder recovery chamber where the powder material is separated from the air. The filtered or "clean" air is drawn through the cartridge filters into a clean air chamber from which it then passes through final filters before being returned to atmosphere. The powder collected within the powder recovery chamber is either scrapped, stored or circulated to the spray guns for spraying. Examples of powder recovery systems of this general type are disclosed, for example, in U.S. Pat. No. 4,409,009 to Lissy; U.S. Pat. No. 4,498,913 to Tank, et al.; and U.S. Pat. No. 4,723,505 to Wilson, et al., all of which are owned by the assignee of this invention.
The popularity of powder coating material has brought with it increasing demands on powder booth manufacturers to provide systems which are versatile, economic and efficient. One customer concern which has not been satisfactorily met involves obtaining a uniform air flow within the booth interior, especially in relatively large volume booths. In many spray booths, the powder recovery system includes a single exhaust fan module connected to a single filter unit or module. Depending upon the location of the exhaust fan and filter unit relative to the powder spray guns, the efficiency of collection of the oversprayed powder may be less than desired because the flow of air within the booth may not be uniform from the inlet end to the outlet end of the booth. For example, many powder spray booth employ groups of spray guns located at different positions therealong which are manually or automatically operated to obtain complete coverage of articles passing through the booth. If the fan module and filter module are located in the center or at one of the ends of the booth, recovery of oversprayed powder from those areas of the booth furthest from the fan module and/or filter unit may not be as efficient as desired and/or as efficient as the area directly beneath the fan. Further, the fan module may not provide an even flow of air throughout the length of the booth which, in turn, adversely affects the transfer efficiency of the powder material, i.e., the effectiveness with which the spray guns apply the powder material onto articles passing through the booth. While this problem can be addressed to some extent by providing a number of separate fan modules and associated filter units at intervals along the length of the booth, such a solution is expensive both in terms of initial investment and maintenance of the system.
Another concern of end users of powder spray booths is to be able to rapidly change from one color of coating material to another with a minimum of downtime. In changing from one color of powder coating material to another, the interior of the spray booth must be completely cleaned of the old color powder material before powder material of a different color can be employed or "speckling" will occur on the new parts to be coated wherein the old and new colors are intermixed. Although some production coating applications require the use of a single color for long periods of time, many smaller volume operations and customers producing customized articles have a need for frequent color changes on a daily basis. Moreover, the economics of such smaller volume operations require that the different colored coating materials be collected for reuse rather than being thrown away as scrap.
This problem has been addressed to some extent in the powder spray booth systems disclosed in U.S. Pat. Nos. 4,498,913 and 4,723,505 mentioned above. In systems of this type, self-contained, cartridge filtering units are moved into position with respect to the interior of a spray booth in preparation for a coating operation involving one color of coating material, and then the filter units are subsequently removed from the booth when the spraying operation for that color is terminated. In order to resume spraying with a new color, the interior of the spray booth is cleaned and a new filter module is moved into position at the spray booth and connected with a fan module permanently associated with the booth.
While systems of this type provide for increased efficiency in effecting a change from one color of coating material to another, they nevertheless require the customer to inventory a separate filter module for each color to be utilized. If a particular business routinely uses many different colors for a particular type or group of articles, the expense of purchasing a large number of separate filter modules, and providing the space to store them, can be prohibitive.